1. Field of the Invention
The present invention relates to apparatus and methods for compressing and manipulating audio data.
2. Description of the Prior Art
For some listeners with sensorineural hearing loss in the high frequency or other frequency ranges, providing audibility of the speech signal in the frequency regions of hearing loss is not effective. These listeners are unsuccessful users of hearing aids.
It is possible to determine the specific frequency regions in which users are unable to use amplified speech, using a measurement technique known as correlational analysis.
The idea of frequency lowering speech is known, but has not thus far been successful. This is because if, in the process of frequency lowering speech, the important cues of speech recognition are transformed into a new form, recognition will be degraded or, at best, require large amounts of training for listeners to learn to use the new cues. Several types of devices such as frequency transposers and vocoders have been tried for hearing impaired listeners with little success. These devices typically shift a band of high frequencies by a fixed number of Hertz to lower frequencies using amplitude modulation techniques or the like. Often the shifted band is mixed with the original low frequency signal. This produces an unnatural speech signal which is not typically useful for hearing impaired individuals.
An example of a commercially available hearing aid which attempts to move sound signals into the frequency range that can be heard by the hearing aid wearer, to increase the wearer""s comprehension of speech and other sounds, accomplishes this task by compressing the audio signal in the time domain. The TranSonic(trademark) Model FT-40 MK II hearing aid, by AVR Communications Ltd. slows down the audio signal to lower its frequency, and then a xe2x80x9crecirculationxe2x80x9d circuit recycles the signal from the storage device back to the input of the storage device to mix with later signals. Other hearing aids have used correlational analysis to process different parts of the audio spectrum differently, according to linear predictive coding or the like.
Human listeners are quite accustomed to recognizing at least one type of frequency compressed speech. The variation in sizes of the vocal apparatus between various speakers and speaker types (e.g. males, females, and children) produces speech that has different frequency contents. Yet most listeners easily adapt to different talkers, and recognition is relatively unaffected. One important unifying characteristic across various individual speakers is that the ratios between the frequencies of the vocal tract resonances (formant peaks) are relatively constant. In other words, the frequency differences between speakers can be represented as proportional differences in formant peaks, whereby each frequency is shifted upward or downward by a fixed multiplicative factor. Thus, proportionally frequency lowering or compression can compress the frequency of a speech signal into the usable portion of the hearing range, while retaining recognition. Similarly, proportionally compressing the audio signal and shifting it into a higher portion of the sound spectrum can offer increased recognition to individuals with hearing deficits in lower frequency ranges.
A need remains in the art for apparatus and methods to provide an understandable audio signal to listeners who have hearing loss in particular frequency ranges, by proportionally compressing the audio signal.
It is an object of the present invention to provide an understandable audio signal to listeners who have hearing loss in particular frequency ranges by proportionally compressing the audio signal. The present invention achieves this objective by maintaining the spectral shape of the audio signal, while scaling its spectrum in the frequency domain, via frequency compression, and transposing its spectrum in the frequency domain, via frequency shifting.